One of the important safety concerns that arises in the use of a pump to intravenously administer drugs to a patient is the need to insure that the medication is reaching the patient at the desired rate. Although medical personnel properly set the controls of an infusion pump to supply medication to the patient at the prescribed rate, the lines to and from the pump may become kinked or otherwise obstructed, for example, due to the patient rolling over onto a line. Many drug infusion pumps have fail-safe mechanisms to prevent drugs from being delivered to a patient at an excessive (e.g., free flow) rate, but only a few are provided with means for detecting a blockage in the delivery or supply lines. Pumps that detect an obstruction in the delivery line usually monitor pressure at the pump output port. Blockage of fluid flow from the pump is detected when the pressure at the pump output port increases to a level that exceeds a predetermined threshold. For detecting an obstruction of the supply line that leads from the drug container to the pump, or for detecting that the drug container is empty, some pumps use drip detectors installed in the supply line immediately upstream of the pump inlet. However, connection of the drip detector adds to the time required to set up a drug infusion system, requires additional hardware, and makes the system more complex to use.
Another potential problem with this type of system concerns the infusion of excessive amounts of air into the patient's circulatory system. To prevent this condition, some infusion pumps include an air-in-line sensor that detects air bubbles larger than a specified size, e.g. larger than 100 microliters, since such bubbles can produce an air embolism in the patient's circulatory system that may be harmful. Air-in-line detectors typically monitor the fluid output of the pump using a matched resonant frequency ultrasonic piezoelectric transmitter and receiver that are disposed on opposite sides of a fluid passage in the pump. The ultrasonic piezoelectric transmitter applies an ultrasonic signal to one side of the fluid passage and the receiver monitors the level of that signal on the other side of the passage. Air bubbles in the fluid passage larger than the prescribed maximum size attenuate the ultrasonic signal that reaches the receiver, causing its output signal to fall below a predefined minimum level. A pump control circuit responds to the reduced output signal level caused by a large air bubble and shuts the pump off to prevent the air bubble from being infused into the patient. Additionally, an alarm is sounded to alert medical personnel that the pump has ceased operation, so that the problem causing the air bubbles can be corrected and operation of the pump restored to normal.
In addition to detecting air bubbles larger than a predefined size in the fluid administered using an infusion pump, an air-in-line sensor should ideally monitor the percentage of air being infused in the fluid, since many smaller air bubbles can coalesce to form a dangerous air embolism in the patient's circulatory system. For example, if the air-in-line sensor detects that more than ten (10) percent of the infused fluid comprises air, the pump should be stopped and an alarm sounded so that the condition causing the small air bubbles can be corrected. However, air-in-line sensors that must be calibrated to respond to a wide range of air bubble sizes are more likely to produce false alarms and to be less accurate in determining the percentage of air infused in the form of small bubbles. It would be preferable to use a different mechanism to prevent relatively larger volumes of air from being infused into a patient.
In consideration of each of the above-described problems, it is an object of this invention to detect whether fluid is flowing through a positive displacement pump. It is a further object to use a sensor integral to the pump to detect that a supply container for the fluid is empty, or to detect that a supply line from the container is blocked, preventing the fluid reaching the inlet of the pump. Yet a further object is to detect the presence of a substantial volume of air or other gaseous fluid in a pumping chamber of the pump and prevent the volume of gaseous fluid being forced from the pump. A still further object is to provide an alarm if the pump is operating, but is not pumping fluid from the pumping chamber. These and other objects and advantages of the present invention will become apparent from the attached drawings and the Description of the Preferred Embodiments that follows.